The present invention relates to "buried-resistance" semiconductor devices, more particularly to those devices used in integrated circuits as resistors having a high resistance and very small bulk, or used as constant-current generators.
The devices, also described in the technical literature as "tunnel-resistance" devices, are normally made by forming a monocrystalline silicon substrate having a first type of conductivity, generally P type, growing an epitaxial layer having a second type of conductivity, generally N type, on the substrate, and insulating a portion of the epitaxial layer by an insulating region extending from the substrate to the surface of the epitaxial layer, the insulation region having the first type of conductivity. Two terminals suitably spaced apart are secured to the surface of the epitaxial layer in the area bounded by the insulating region.
The resistivity and thickness of the epitaxial layer and the distance between the terminals determine the total electrical resistance between the two terminals when a voltage is applied therebetween.
When the applied voltage increases, the resistance remains constant, i.e. there is an ohmic-type relationship between the voltage and the current flowing in the device up to a threshold value of the voltage, above which the current remains constant when the applied voltage increases, so that the device acts as a constant current generator.
The reasons this happens is that a depletion region of majority carriers forms in the epitaxial layer at the junction with the substrate, which is maintained at a reference voltage, the extent of the depletion region increasing with the applied voltage, particularly in the region underneath the terminal at the higher potential.
When the voltage reaches a threshold value V.sub.PINCH corresponding to the state of complete depletion of the epitaxial layer, the current in the device cannot be increased and therefore remains constant even when the applied voltage increases.